Resource Timing (original) (raw)

This specification defines an interface for web applications to access the complete timing information for resources in a document.

Introduction

User latency is an important quality benchmark for Web Applications. While JavaScript-based mechanisms can provide comprehensive instrumentation for user latency measurements within an application, in many cases, they are unable to provide a complete end-to-end latency picture. This document introduces the PerformanceResourceTiming interface to allow JavaScript mechanisms to collect complete timing information related to resources on a document. Navigation Timing 2 [[NAVIGATION-TIMING-2]] extends this specification to provide additional timing information associated with a navigation.

For example, the following JavaScript shows a simple attempt to measure the time it takes to fetch a resource:

    <!doctype html>
    <html>
      <head>
      </head>
      <body onload="loadResources()">
        <script>
            function loadResources()
            {
              var start = new Date().getTime();
              var image1 = new Image();
              var resourceTiming = function() {
                  var now = new Date().getTime();
                  var latency = now - start;
                  alert("End to end resource fetch: " + latency);
              };

              image1.onload = resourceTiming;
              image1.src = 'https://www.w3.org/Icons/w3c_main.png';
            }
        </script>
        <img src="https://www.w3.org/Icons/w3c_home.png">
      </body>
    </html>

Though this script can measure the time it takes to fetch a resource, it cannot break down the time spent in various phases. Further, the script cannot easily measure the time it takes to fetch resources described in markup.

To address the need for complete information on user experience, this document introduces the PerformanceResourceTiming interface. This interface allows JavaScript mechanisms to provide complete client-side latency measurements within applications. With this interface, the previous example can be modified to measure a user's perceived load time of a resource.

The following script calculates the amount of time it takes to fetch every resource in the page, even those defined in markup. This example assumes that this page is hosted on https://www.w3.org. One could further measure the amount of time it takes in every phase of fetching a resource with the PerformanceResourceTiming interface.

    <!doctype html>
    <html>
      <head>
      </head>
      <body onload="loadResources()">
        <script>
          function loadResources()
          {
              var image1 = new Image();
              image1.onload = resourceTiming;
              image1.src = 'https://www.w3.org/Icons/w3c_main.png';
          }

          function resourceTiming()
          {
              var resourceList = window.performance.getEntriesByType("resource");
              for (i = 0; i < resourceList.length; i++)
              {
                  if (resourceList[i].initiatorType == "img")
                  {
                    alert("End to end resource fetch: " + (resourceList[i].responseEnd - resourceList[i].startTime));
                  }
              }
          }
        </script>
        <img id="image0" src="https://www.w3.org/Icons/w3c_home.png">
      </body>
    </html>

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("MUST", "SHOULD", "MAY", etc) used in introducing the algorithm.

Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on user agents.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

Terminology

The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo.

Throughout this work, all time values are measured in milliseconds since the start of navigation of the document [[HR-TIME]]. For example, the start of navigation of the document occurs at time 0.

This definition of time is based on the High Resolution Time specification [[HR-TIME]] and is different from the definition of time used in the Navigation Timing specification [[NAVIGATION-TIMING-2]], where time is measured in milliseconds since midnight of January 1, 1970 (UTC).

Introduction

The PerformanceResourceTiming interface facilitates timing measurement of [=fetch|fetched=] [=http(s) scheme|http(s)=] resources. For example, this interface is available for {{XMLHttpRequest}} objects [[XHR]], HTML elements [[HTML]] such as [^iframe^], [^img^], [^script^], [^object^], [^embed^] and [^link^] with the link type of [^link/rel/stylesheet^], SVG elements [[SVG11]] such as svg, and {{EventSource}}.

Resources Included in the PerformanceResourceTiming Interface

This section is non-normative.

Resource [=Request=]s [=fetch=]ed by a non-null [=request/client=] are included as PerformanceResourceTiming objects in the [=request/client=]'s [=environment settings object/global object=]'sPerformance Timeline, unless excluded from the timeline as part of the [=fetch|fetching process=]. Resources that are retrieved from HTTP cache are included as PerformanceResourceTiming objects in thePerformance Timeline. Resources for which the [=fetch=] was initiated, but was later aborted (e.g. due to a network error) are included asPerformanceResourceTiming objects in the Performance Timeline, with their start and end timing.

Examples:

The PerformanceResourceTiming Interface

      [Exposed=(Window,Worker)]
      interface PerformanceResourceTiming : PerformanceEntry {
          readonly attribute DOMString initiatorType;
          readonly attribute DOMString deliveryType;
          readonly attribute ByteString nextHopProtocol;
          readonly attribute DOMHighResTimeStamp workerStart;
          readonly attribute DOMHighResTimeStamp redirectStart;
          readonly attribute DOMHighResTimeStamp redirectEnd;
          readonly attribute DOMHighResTimeStamp fetchStart;
          readonly attribute DOMHighResTimeStamp domainLookupStart;
          readonly attribute DOMHighResTimeStamp domainLookupEnd;
          readonly attribute DOMHighResTimeStamp connectStart;
          readonly attribute DOMHighResTimeStamp connectEnd;
          readonly attribute DOMHighResTimeStamp secureConnectionStart;
          readonly attribute DOMHighResTimeStamp requestStart;
          readonly attribute DOMHighResTimeStamp finalResponseHeadersStart;
          readonly attribute DOMHighResTimeStamp firstInterimResponseStart;
          readonly attribute DOMHighResTimeStamp responseStart;
          readonly attribute DOMHighResTimeStamp responseEnd;
          readonly attribute unsigned long long  transferSize;
          readonly attribute unsigned long long  encodedBodySize;
          readonly attribute unsigned long long  decodedBodySize;
          readonly attribute unsigned short responseStatus;
          readonly attribute RenderBlockingStatusType renderBlockingStatus;
          readonly attribute DOMString contentType;
          [Default] object toJSON();
      };

A PerformanceResourceTiming has an associated DOMStringinitiator type.

A PerformanceResourceTiming has an associated DOMStringdelivery type.

A PerformanceResourceTiming has an associated DOMStringrequested URL.

A PerformanceResourceTiming has an associated DOMStringcache mode (the empty string, "local", or "validated").

A PerformanceResourceTiming has an associated [=fetch timing info=] timing info.

A PerformanceResourceTiming has an associated [=response body info=] resource info.

A PerformanceResourceTiming has an associated status response status.

A PerformanceResourceTiming has an associated {{RenderBlockingStatusType}} render-blocking status.

When toJSON is called, run the [=default toJSON steps=] for {{PerformanceResourceTiming}}.

initiatorType getter steps are to return the initiator type for this.

`initiatorType` returns one of the following values:

The setting of `initiatorType` is done at the different places where a resource timing entry is reported, such as the [=fetch=] standard.

deliveryType getter steps are to return the delivery type for this.

`deliveryType` returns one of the following values:

This is expected to be expanded by future updates to this specification, e.g. to describe consuming preloaded resources and prefetched navigation requests.

The workerStart getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/final service worker start time=] and the relevant global object for this. See [=/HTTP fetch=] for more info.

The redirectStart getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/redirect start time=] and the relevant global object forthis. See [=/HTTP-redirect fetch=] for more info.

The redirectEnd getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/redirect end time=] and the relevant global object forthis. See [=/HTTP-redirect fetch=] for more info.

The fetchStart getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/post-redirect start time=] and the relevant global object for this. See [=/HTTP fetch=] for more info.

The domainLookupStart getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/final connection timing info=]'s [=connection timing info/domain lookup start time=] and the relevant global object forthis. See Recording connection timing info for more info.

The domainLookupEnd getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/final connection timing info=]'s [=connection timing info/domain lookup end time=] and the relevant global object forthis. See Recording connection timing info for more info.

The connectStart getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/final connection timing info=]'s [=connection timing info/connection start time=] and the relevant global object for this. See Recording connection timing info for more info.

The connectEnd getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/final connection timing info=]'s [=connection timing info/connection end time=] and the relevant global object forthis. See Recording connection timing info for more info.

The secureConnectionStart getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/final connection timing info=]'s [=connection timing info/secure connection start time=] and the relevant global object forthis. See Recording connection timing info for more info.

The nextHopProtocol getter steps are to [=/isomorphic decode=] this's [=PerformanceResourceTiming/timing info=]'s [=fetch timing info/final connection timing info=]'s [=connection timing info/ALPN negotiated protocol=]. See Recording connection timing info for more info.

Issue 221 suggests to remove support for nextHopProtocol, as it can reveal details about the user's network configuration.

The requestStart getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/final network-request start time=] and the relevant global object for this. See [=/HTTP fetch=] for more info.

The firstInterimResponseStart getter steps are toconvert fetch timestamp for this's timing info's [=fetch timing info/first interim network-response start time=] and the relevant global object for this. See [=/HTTP fetch=] for more info.

The finalResponseHeadersStart getter steps are toconvert fetch timestamp for this's timing info's [=fetch timing info/final network-response start time=] and the relevant global object for this. See [=/HTTP fetch=] for more info.

The responseStart getter steps are to return this's {{PerformanceResourceTiming/firstInterimResponseStart}} if it is not 0; Otherwise this's {{PerformanceResourceTiming/finalResponseHeadersStart}}.

The responseEnd getter steps are to convert fetch timestamp for this's timing info's [=fetch timing info/end time=] and the relevant global object for this. See [=/fetch=] for more info.

The encodedBodySize getter steps are to returnthis's resource info's [=response body info/encoded size=].

The decodedBodySize getter steps are to returnthis's resource info's [=response body info/decoded size=].

The transferSize getter steps are to perform the following steps:

  1. If this's cache mode is "local", then return 0.
  2. If this's cache mode is "validated", then return 300.
  3. Return this's response body info's [=response body info/encoded size=] plus 300.
    The constant number added to `transferSize` replaces exposing the total byte size of the HTTP headers, as that may expose the presence of certain cookies. See this issue.

The responseStatus getter steps are to returnthis's response status.

`responseStatus` is determined in [=Fetch=]. For a cross-originno-cors request it would be 0 because the response would be an opaque filtered response.

The contentType getter steps are to return this'sresource info's [=response body info/content type=].

The renderBlockingStatus getter steps are to returnblocking ifthis's timing info's [=fetch timing info/render-blocking=] is true; otherwisenon-blocking.

A user agent implementing PerformanceResourceTiming would need to include "resource" in {{PerformanceObserver/supportedEntryTypes}}. This allows developers to detect support for Resource Timing.

RenderBlockingStatusType enum

        enum RenderBlockingStatusType {
            "blocking",
            "non-blocking"
        };

The values are defined as follows:

blocking

The resource can potentially block rendering.

non-blocking

The resource will not block rendering.

Extensions to the Performance Interface

The user agent MAY choose to limit how many resources are included asPerformanceResourceTiming objects in the Performance Timeline [[PERFORMANCE-TIMELINE-2]]. This section extends thePerformance interface to allow controls over the number ofPerformanceResourceTiming objects stored.

The recommended minimum number of PerformanceResourceTiming objects is 250, though this may be changed by the user agent.setResourceTimingBufferSize can be called to request a change to this limit.

Each ECMAScript global environment has:

partial interface Performance { undefined clearResourceTimings (); undefined setResourceTimingBufferSize (unsigned long maxSize); attribute EventHandler onresourcetimingbufferfull; };

The Performance interface is defined in [[HR-TIME]].

The method clearResourceTimings runs the following steps:

  1. Remove all PerformanceResourceTiming objects in theperformance entry buffer.
  2. Set resource timing buffer current size to 0.

The setResourceTimingBufferSize method runs the following steps:

  1. Set resource timing buffer size limit to the_maxSize_ parameter. If the maxSize parameter is less than resource timing buffer current size, noPerformanceResourceTiming objects are to be removed from theperformance entry buffer.

The attribute onresourcetimingbufferfull is the event handler for the resourcetimingbufferfull event described below.

To check if can add resource timing entry, run the following steps:

  1. If resource timing buffer current size is smaller than resource timing buffer size limit, return true.
  2. Return false.

To add a PerformanceResourceTiming entry new entry into the performance entry buffer, run the following steps:

  1. If can add resource timing entry returns true andresource timing buffer full event pending flag is false, run the following substeps:
    1. Add new entry to the performance entry buffer.
    2. Increase resource timing buffer current size by 1.
    3. Return.
  2. If resource timing buffer full event pending flag is false, run the following substeps:
    1. Set resource timing buffer full event pending flag to true.
    2. Queue a task on the performance timeline task source to run fire a buffer full event.
  3. Add new entry to the resource timing secondary buffer.
  4. Increase resource timing secondary buffer current size by 1.

To copy secondary buffer, run the following steps:

  1. While resource timing secondary buffer is not empty andcan add resource timing entry returns true, run the following substeps:
    1. Let entry be the oldestPerformanceResourceTiming in resource timing secondary buffer.
    2. Add entry to the end of performance entry buffer.
    3. Increment resource timing buffer current size by 1.
    4. Remove entry from resource timing secondary buffer.
    5. Decrement resource timing secondary buffer current size by 1.

To fire a buffer full event, run the following steps:

  1. While resource timing secondary buffer is not empty, run the following substeps:
    1. Let number of excess entries before be resource timing secondary buffer current size.
    2. If can add resource timing entry returns false, then [=fire an event=] named resourcetimingbufferfull at the {{Performance}} object.
    3. Run copy secondary buffer.
    4. Let number of excess entries after be resource timing secondary buffer current size.
    5. If number of excess entries before is lower than or equals number of excess entries after, then remove all entries from resource timing secondary buffer, setresource timing secondary buffer current size to 0, and abort these steps.
  2. Set resource timing buffer full event pending flag to false.
    This means that if the resourcetimingbufferfull event handler does not add more room in the buffer than it adds resources to it, excess entries will be dropped from the buffer. Developers should make sure thatresourcetimingbufferfull event handlers callclearResourceTimings or extend the buffer sufficiently (by callingsetResourceTimingBufferSize).

Cross-origin Resources

As detailed in [=Fetch=], requests for cross-origin resources are included as PerformanceResourceTiming objects in thePerformance Timeline. If the timing allow check algorithm fails for a cross-origin resource, the entry will be an [=create an opaque timing info|opaque entry=]. Such entries have most of their attributes masked in order to prevent leaking cross-origin data that isn't otherwise exposed. So, for an [=create an opaque timing info|opaque entry=], the following attributes will be set to zero: {{PerformanceResourceTiming/redirectStart}}, {{PerformanceResourceTiming/redirectEnd}}, {{PerformanceResourceTiming/workerStart}}, {{PerformanceResourceTiming/domainLookupStart}}, {{PerformanceResourceTiming/domainLookupEnd}}, {{PerformanceResourceTiming/connectStart}}, {{PerformanceResourceTiming/connectEnd}}, {{PerformanceResourceTiming/requestStart}}, {{PerformanceResourceTiming/firstInterimResponseStart}}, {{PerformanceResourceTiming/finalResponseHeadersStart}}, {{PerformanceResourceTiming/responseStart}}, {{PerformanceResourceTiming/secureConnectionStart}}, {{PerformanceResourceTiming/transferSize}}, {{PerformanceResourceTiming/encodedBodySize}}, and {{PerformanceResourceTiming/decodedBodySize}}. Further, the {{PerformanceResourceTiming/nextHopProtocol}} attribute will be set to the empty string.

Server-side applications may return the Timing-Allow-Origin HTTP response header to allow the User Agent to fully expose, to the document origin(s) specified, the values of attributes that would have been zero due to those cross-origin restrictions.

Timing-Allow-Origin Response Header

The Timing-Allow-Origin HTTP response header field can be used to communicate a policy indicating origin(s) that may be allowed to see values of attributes that would have been zero due to the cross-origin restrictions. The header's value is represented by the following ABNF [[RFC5234]] (using List Extension, [[RFC9110]]):

Timing-Allow-Origin = 1#( origin-or-null / wildcard )

The sender MAY generate multiple Timing-Allow-Origin header fields. The recipient MAY combine multipleTiming-Allow-Origin header fields by appending each subsequent field value to the combined field value in order, separated by a comma.

The user agent MAY still enforce cross-origin restrictions and set transferSize, encodedBodySize, and decodedBodySize attributes to zero, even with Timing-Allow-Origin HTTP response header fields. If it does, it MAY also set deliveryType to "".

The Timing-Allow-Origin headers are processed inFETCH to compute the attributes accordingly.

The Timing-Allow-Origin header may arrive as part of a cached response. In case of cache revalidation, according to RFC 7234, the header's value may come from the revalidation response, or if not present there, from the original cached resource.

Issues 222 and223 suggest to remove wildcard support from Timing-Allow-Origin in order to restrict its use.

IANA Considerations

This section registers Timing-Allow-Origin as a Provisional Message Header.

Header field name:

Timing-Allow-Origin

Applicable protocol:

http

Status:

provisional

Author/Change controller:

W3C

Specification document:

Resource Timing Attributes

This section is non-normative.

The following graph illustrates the timing attributes defined by the PerformanceResourceTiming interface. Attributes in parenthesis may not be available when [=fetch|fetching=] cross-origin resources. User agents may perform internal processing in between timings, which allow for non-normative intervals between timings.

This figure illustrates the timing attributes defined by thePerformanceResourceTiming interface. Attributes in parenthesis indicate that they may not be available if the resource fails the timing allow check algorithm.

Resource Timing attributes

Creating a resource timing entry

To mark resource timing given a [=/fetch timing info=] |timingInfo|, a DOMString |requestedURL|, a DOMString |initiatorType| a global object |global|, a string |cacheMode|, a [=/response body info=] |bodyInfo|, a status |responseStatus|, and an optional [=string=] |deliveryType| (by default, the empty string), perform the following steps:

  1. Create a PerformanceResourceTiming object |entry| in |global|'s [=global object/realm=].
  2. Setup the resource timing entry for |entry|, given |initiatorType|, |requestedURL|, |timingInfo|, |cacheMode|, |bodyInfo|, |responseStatus|, and |deliveryType|.
  3. Queue |entry|.
  4. [=Add a PerformanceResourceTiming entry|Add=] |entry| to |global|'s performance entry buffer.

To setup the resource timing entry forPerformanceResourceTiming |entry| given DOMString |initiatorType|, DOMString |requestedURL|, [=/fetch timing info=] |timingInfo|, a DOMString |cacheMode|, a [=response body info=] |bodyInfo|, a status |responseStatus|, and an optional DOMString |deliveryType| (by default, the empty string), perform the following steps:

  1. Assert that |cacheMode| is the empty string, "local", or "validated".
  2. Let |global| be |entry|'s [=relevant global object=].
  3. [=initialize a PerformanceEntry|Initialize=] |entry| given the result of converting |timingInfo|'s [=fetch timing info/start time=] given |global|, "resource", |requestedURL|, and the result ofconverting |timingInfo|'s [=fetch timing info/end time=] given |global|.
  4. Set |entry|'s initiator type to |initiatorType|.
  5. Set |entry|'s requested URL to |requestedURL|.
  6. Set |entry|'s timing info to |timingInfo|.
  7. Set |entry|'s response body info to |bodyInfo|.
  8. Set |entry|'s cache mode to |cacheMode|.
  9. Set |entry|'s response status to |responseStatus|.
  10. If |deliveryType| is the empty string and |cacheMode| is not, then set |deliveryType| to "cache".
  11. Set |entry|'s delivery type to |deliveryType|.

To convert fetch timestamp given {{DOMHighResTimeStamp}} |ts| and global object |global|, do the following:

  1. If |ts| is zero, return zero.
  2. Otherwise, return the [=relative high resolution coarse time=] given |ts| and |global|.

Security Considerations

The PerformanceResourceTiming interface exposes timing information for a resource to any web page or worker that has requested that resource. To limit the access to thePerformanceResourceTiming interface, the same origin policy is enforced by default and certain attributes are set to zero, as described in [=/HTTP fetch=]. Resource providers can explicitly allow all timing information to be collected for a resource by adding the Timing-Allow-Origin HTTP response header, which specifies the domains that are allowed to access the timing information.

Privacy Considerations

Statistical fingerprinting is a privacy concern where a malicious web site may determine whether a user has visited a third-party web site by measuring the timing of cache hits and misses of resources in the third-party web site. Though the PerformanceResourceTiming interface gives timing information for resources in a document, the load event on resources can already measure timing to determine cache hits and misses in a limited fashion, and the cross-origin restrictions in [=/HTTP Fetch=] prevent the leakage of any additional information.

Acknowledgments

Thanks to Anne Van Kesteren, Annie Sullivan, Arvind Jain, Boris Zbarsky, Darin Fisher, Jason Weber, Jonas Sicking, James Simonsen, Karen Anderson, Kyle Scholz, Nic Jansma, Philippe Le Hegaret, Sigbjørn Vik, Steve Souders, Todd Reifsteck, Tony Gentilcore, William Chan, and Alex Christensen for their contributions to this work.